1. Field of the Invention
The present invention relates to electromagnetic inspection of an object of electrically conducting material.
2. Description of Related Art
It is known to measure the wall thickness of a conductive object by using eddy currents, bee for example International patent application publication No. WO 98/02 714 and European patent specification No. 321 112. When measuring an unknown wall thickness, there are three wall thicknesses, (1) the actual wall thickness or true wall thickness, (2) the measured wall thickness (before correction) and (3) the corrected wall thickness. Correcting the measured wall thickness must be done in order to get a wall thickness that is nearer to the actual wall thickness than the measured wall thickness before correction.
In the present electromagnetic inspection method, a probe is used that comprises a transmitter coil for inducing eddy currents in the object, and a receiver system for providing a signal indicative of the strength of the electromagnetic field generated by the eddy currents or of changes in the strength of this electromagnetic field. In particular, the present invention relates to measuring a wall thickness and correcting the measured wall thickness for unknown variations in lift-off. Lift-off is the distance between the probe and the near surface of the object, which is the surface nearest to the probe in contrast to the far surface that is at the opposite side of the object.
Examples of objects that can suitably be inspected with the method according to the present invention are metal plates or walls of container means, such as pipes, vessels or containers, which have a radius of curvature that is larger than the thickness. The electrically conducting material can be any electrically conducting material, for example carbon steel or stainless steel. A further application of the method of the present invention is that the thickness of a layer of insulation can be measured.
It is an object of the present invention to make measurements of the thickness of an object more accurate. To this end the present invention provides a method of measuring the wall thickness of an electrically conductive object using a probe, which probe comprises a transmitter coil for inducing eddy currents in the object, and a receiver system for providing a signal indicative of the strength of the electromagnetic field generated by the eddy currents or of changes in the strength of this electromagnetic field, wherein the receiver system comprises a first receiver and a second receiver that is spaced apart from the first receiver, which method comprises the steps of:
(a) arranging the probe near the object at a distance from the near surface of the object;
(b) inducing transient eddy currents in the object by activating the transmitter and recording the signal of the receivers with time, wherein Vl(t) is the signal of the first receiver with time (t) and Vu(t) is the signal of the second receiver with time (t);
(c) measuring the wall thickness from a characteristic of one of the signals;
(d) calculating a characteristic value, "PHgr", from a combination of Vl(t) and Vu(t); and
(e) correcting the measured wall thickness for the distance between the probe and the near surface of the object using a predetermined relationship between the wall thickness and the characteristic value for different values of the distance between the probe and the near surface